Indoor accelerated weathering test apparatus are known to test the accelerated aging characteristics of painted surfaces, fabrics, plastic sheeting and other materials. Such testing is accomplished by exposing the materials to be tested to high intensity radiation from an artificial light source that approximates sunlight, under conditions of controlled and sometimes high temperature and/or humidity.
In a natural outdoor environment, heat, light and moisture combine to synergistically cause optical, mechanical and chemical changes in products which are exposed to such outdoor weathering conditions. Generally, the test apparatus of the present invention and the prior art can be used to obtain such weathering data on an accelerated time basis, to permit product manufacturers to gain information as to how their products will stand up to weathering conditions over the months or years.
Typically, an accelerated weathering test apparatus may use air which circulates through the system to control the temperature of samples being tested, so that they are not underheated or overheated by heater or radiation source which may be present, typically a high-intensity plasma lamp such as a xenon lamp. It is desirable for the samples being tested to be exposed to precisely predetermined conditions, to permit more accurate comparison between various testing runs and so that the weathering conditions provided by the test apparatus can be accurately predetermined and thus recreated when desired for comparison of various samples over the years.
In known accelerated weathering test apparatus, a rotatable rack for carrying the samples to be tested surrounds a light source, often a xenon lamp, which emits irradiation having a substantial ultraviolet component. The rack is rotated typically about one revolution per minute, to avoid any systematic differences of positioning of the samples in the system. Also, the typical level of irradiation imposed on the samples is approximately one SUN, which is defined in The Society of Automotive Engineers J-1885 weathering testing method to be 0.55 watt per square meter at 340 nanometers ultraviolet radiation.
Other known accelerated weathering test apparatus further accelerate the aging of materials by exposing such materials to an irradiance level that is higher than one SUN, for example two SUNs (or about 1.1 watts per square meter in accordance with the previous definition). It has been noted that at such higher light intensities, the irregularity of light irradiance around the rack at the area of the samples becomes larger, contributing to sample temperature variations. As a result, the samples may be affected in their testing program by these variables.
Other known accelerated weathering test apparatus monitor and control irradiance of the light source only at three discrete points of the light source SPD. Namely, prior art test apparatus measure light source irradiance only at 340 nanometers (“nm”), 420 nm and 300–400 nm. Measurements are made by a fixed band-pass optical filter and associated closed loop feedback electronics. Standard test methods specify one of the three control points and are not user selectable. These known test monitoring and controlling methods are particularly disadvantageous for several reasons. For example, test specimen materials currently under development are sensitive to, age or degrade as a result of exposure to irradiance from the light source at specific wavelengths other than the set standard. In current instruments it is not possible to control the wavelength of maximum or critical sensitivity for specific materials. Further, the SPD of the light source changes as the light source and inner and outer filters age over time. Again, with a static irradiance control wavelength the optimum accelerated weathering cannot be achieved. As a result, the reliability of the test specimens is affected in their respective testing programs by these variables.
Calibration of known accelerated weathering test apparatus is also cumbersome, time consuming and introduces considerable margin for error into the test results for a client accelerated weathering test apparatus. Prior art calibration schemes are directed to the steps of: calibrating a spectroradiometer from a 1000 watt Tungsten calibration standard; measuring a standard factory light source with the spectroradiometer and assigning a calibration value; calibrating a factory accelerated weathering test apparatus radiometer by operation with the standard factory light source and adjusting radiometer gain in accordance with the calibration value; operating factory accelerated weathering test apparatus with a client standard light source and assigning calibration values based on radiometer readings; and operating a client accelerated weathering test apparatus with the client standard light source and adjusting radiometer gain of client test apparatus to match calibration values. As a result, the possibilities for uncertainties produced by known prior art apparatus is sizeable and vast. Even if the factory executes each of its steps flawlessly, there are still opportunities for the client to make errors. Accordingly, the test specimens are affected in their respective testing program by these variables.
One known weathering apparatus includes a radiation measuring device. A portion of radiation used for testing is guided to the measuring device. The guided radiation is spectrally dispersed so that intensity and/or dosage may be measured by selected diodes at discrete points on the SPD. The radiation detector consists of an array of photodiodes assigned to monitor preselected discrete wave lengths.
Another prior art apparatus for exposing photographic film includes a source of illumination operated at a constant correlated color temperature and intensity. A spectroradiometer takes in light images of the spectrum from 380 nm to 740 nm onto a linear array of thirty-two photodiodes. As a result, the spectral radio meter provides thirty-two signals indicative of the intensity of light in each of the thirty-two uniform which bands together extending from 380 nm to 740 nm. The value of the color temperatures and illuminance for the thirty-two wavelengths nominally at the middle of each of the thirty-two bands are derived from the thirty-two signals from the sensors. From these values, the luminosity of radiant power in color temperature can be derived. The spectroradiometer generates signals indicative of the illuminance and the correlated color temperature, which are transmitted to an automatic control which tests the signals to determine if they are within tolerance. The automatic control and a stepping motor are responsive to signals from the spectroradiometer for adjusting the intensity of a light emitted by the generator. In order to keep color temperature and radiation constant, the distance between the light source and a spherical mirror is altered to adjust the intensity.
Yet another prior art weathering instrument includes a light intensity monitoring and adjusting device including a light guide made of optical fiber, a light receiving section and an adjusting section in a recording instrument. The light guide is configured as a flexible tube containing a bundle of optical fibers which is tri-sected. One end of the light guide is directed toward the lamp and the other, tri-sected, end is connected to the light receiving section. A lens in the light receiving section for each part of the bundle of fibers directs the light to respective light receiving elements, such as photoelectric tubes, through respective filters. The three light receiving elements measure the composition of light at the three fixed, discrete points. One sensor is used to control the intensity of the light and the other two sensors are used to compare what set points to judge the quality of the spectrum.
Still another prior art test apparatus describes a methodology for calibration of a radiometric device with radiation at various intensity levels and spectral distributions. The calibration system includes a light source which emits a beam of light in the direction of a radiometric device for calibrating and/or testing a device. A portion of the light beam is intercepted by the device and another portion of the light beam is intercepted by a detector which is a photodiode. The detector is operated with spectral filtering to view one or more specific spectral bands of interest in the radiation outputted by the light source. The detector provides an output current, via a switch, to a control unit for operating an intensity controller to energize the light source. The current of a single photodetector is asserted to be an accurate predictor of the light intensity within the filtered band for characterizing a linear relationship between photodetector current and intensity.
Therefore, there exists a need in the art for an accelerated weathering test apparatus which overcomes the disadvantages of the prior art, namely: monitoring and controlling a test apparatus with respect to fixed, discrete portions of a light source SPD, inability to calibrate, monitor and control the test apparatus based on the full SPD of a light source, inability to calibrate, monitor and control a test apparatus light source with respect to a user-selectable discrete wavelength, i.e. wavelengths or wavelength range inability to test material sensitivity to different parts of the full SPD, inability to calibrate a test apparatus over a full SPD for a given light source with respect to accepted professional certified standards and inability to monitor changes to the full SPD of a given light source as such light source or associated filters degrade with time.
By the present invention, improvements are provided which increase the accuracy of the calibration, monitoring and control of the test apparatus of this invention. In that the test apparatus can be used to provide accurately predetermined conditions which are substantially predictable and invariant throughout a run and from run to run.